The invention relates to xerographic process control, and more particularly, to the compensation for higher or lower toner concentration levels.
Typically, an electrophotographic process is controlled by adjusting development field, cleaning field, exposure intensity, and toner concentration. An electrostatic voltmeter is used to measure the electrostatic fields. The electrostatic fields are adjusted successively to establish a desired operating range. Voluminous data is collected and analyzed to generate lookup tables in order to bring the density of an image, the developed mass per unit area within prescribed limits.
A common technique for monitoring developed mass per unit area is to artificially create a "test patch" of a predetermined desired density. The actual density of the printing material (toner or ink) in the test patch can then be optically measured to determine the effectiveness of the printing process in placing this printing material on the print sheet.
The optical device for determining the density of toner on the test patch, which is often referred to as a "densitometer", is disposed along the path of the photoreceptor, directly downstream of the development of the development unit. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to charge or discharge as necessary the surface at the location to a predetermined extent.
The test patch is then moved past the developer unit and the toner particles within the developer unit are caused to adhere to the test patch electrostatically. The denser the toner on the test patch, the darker the test patch will appear in optical testing. The developed test patch is moved past a densitometer disposed along the path of the photoreceptor, and the light absorption of the test patch is tested; the more light that is absorbed by the test patch, the denser the toner on the test patch.
In the prior art U.S. Pat. No. 4,348,099 discloses a control system for use in an electrophotographic printing machine. A charge control loop, an illumination control loop, a bias control loop, and a toner dispensing loop are provided. Test patches, an infrared densitometer, and an electrometer are used to measure charge level, exposure intensity, toner concentration, and developer bias.
U.S. Pat. No. 4,553,033 discloses an infrared densitometer for measuring the density of toner particles on a photoconductive surface. A tonal test patch is projected by a test patch generator onto the photoconductive surface. The patch is then developed with toner particles. Infrared light is emitted from the densitometer and reflected back from the test patch. Control circuitry, associated with the densitometer, generated electrical signals proportional to the developer toner mass of the test patch.
U.S. Pat. No. 5,416,564 and U.S. Pat. No. 5,383,005 disclose a current sensing device that generates electrical signals proportional to the current flow between the photoconductive surface and a development station as toner is applied to the photoconductive surface at pre-determined regions or patches. A charging device is controlled in response to the generated signals.
U.S. Pat. No. 5,436,705 discloses an adaptive process control including the use of signals from both a toner area coverage sensor representing a toner reproduction curve and a toner concentration sensor to compensate for image quality due to material aging and environmental changes.
Prior art control processes often rely on various halftone patches to control TC/tribo (Toner Concentration/triboelectricity) and electrostatics in order that image quality outputs such as toner mass per unit area and tone reproduction curve can meet their targets and be maintained. Under normal circumstances and provided that the system is time invariant, such controls may work well.
However, the environmental noise, the customer usage noise, the subsystem design variations, the consumable (toner developer) noise, and interactions between electrostatics and TC/tribo often make the control extremely difficult. This type of control scheme has a strong coupling between the electrostatic actuators and the TC actuator.
With neither an electrostatic voltmeter (ESV) nor a TC sensor, the system depends on time invariant distinct characteristics of the various patches with respect to each and every actuator to control the system and keep the actuators within their normal operating ranges. If the system is not truly time invariant, many system characteristics are no longer unique and distinguishable. Interactions take over and may drive the system to very strange operating spaces.
Furthermore, satisfying the patches alone does not guarantee TMA control which is very important for fusing and many image quality attributes. TMA is a function of the patch RR's (relative reflectance), tribo, the exposure region of the photoreceptor PIDC, cleaning voltage, and the hardware. Again, without an ESV and TC sensor, the consistent control of TMA is difficult. To make matters worse, hardware variations and uncontrollable noises are often excessive.
In essence there are too many unknowns and not enough information (or knowledge) to ensure a robust control system. Current coupled (electrostatics and TC/tribo) control schemes can easily be confused and create an internal compensation problem, that is, the lowering of the electrostatics to compensate for an over tone situation or vice versa.
It would be desirable, therefore, to be able to overcome the above development control difficulties in the prior art.
It is an object of the present invention therefore to provide a new and improved technique for process control, in particular, allowing the setup and control of the TC and tribo characteristics in all environments independent of the process control electrostatics. It is another object of the present invention to provide uncoupled TC/tribo control using the same halftone patches as in existing process controls, in particular, using only a toner area coverage sensor to measure the relative reflectance of test patches. Another object of the present invention is to be able to control TC and automatically compensate to changes in the environment and to control TC and yet be insensitive to a temporary decay of tribo (such as an overnight rest) to avoid TC tone down spiral problems. Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.